Allow any theme to be dark or light, determine it automatically

By definition, "[colors]" is a dark theme and the alternate theme
("[colors2]") is light.

A user who doesn't know about this definition (or about "[colors2]"),
might configure "[colors]" to be a light theme.
This is a reasonable mistake because
a. "colors" is an innocuous name
b. users who failed to run
   "git merge-file ~/.config/foot/foot.ini $old_foot/foot.ini $new_foot/foot.ini"
   after upgrading foot might not have "colors2" in their config.

The wrongly reported color theme (CSI 997) causes issues when apps
use it for selecting colors.  I don't know if any relevant app does,
but learning this cost me some time, and maybe it's a good idea to
address this, even though it's technically a user error.

Solution 1:
Stop responding to CSI 996.
The Contour spec is [ambiguous](https://github.com/contour-terminal/contour/issues/1659#issuecomment-3596983337).
Apps might want to prefer the more widely available OSC 10/11 for detecting dark/light mode.
I think Vim does; and at least NeoVim still uses the Contour protocol to subscribe to notifications.
We'd still send DSR 2031 notifications, however that would work just fine for apps like NeoVim that ignore the payload and only use it as trigger to query for the theme again via OSC 10/11.
(Since we can only switch between two themes, we wouldn't even waste bandwidth.)

Solution 2:
Assuming the themes are only meant for the dark/light mode toggle,
rename them to "colors-dark" and "colors-light",
and maybe report color theme only for those
(and not when the user has the legacy "colors" and "colors2").

Solution 3 (implemented here):
Assuming the themes are intended to be used for things other than
dark/light toggle,
- have foot automatically detect whether the current theme is dark or light
- if needed, allow users to override this (e.g. "colors.is_dark = true")

I guess I have a slight preference for solution 2 because it seems
relatively simple. But I don't know what's the goal.

Apparently switching to dark/light mode at dusk/dawn is a feature
of macOS and there are solutions like darkmon for other OSes, so I
guess dynamic switching is a useful feature in principle.

CHANGELOG.md

@@ -70,6 +70,7 @@
 ## Unreleased
 ### Added
 
+* TODO
 * `toplevel-tag` option (and `--toplevel-tag` command line options to
   `foot` and `footclient`), allowing you to set a custom toplevel
   tag. The compositor must implement the new `xdg-toplevel-tag-v1`

config.c

@@ -25,6 +25,7 @@
 #include "input.h"
 #include "key-binding.h"
 #include "macros.h"
+#include "srgb.h"
 #include "tokenize.h"
 #include "util.h"
 #include "xmalloc.h"
@@ -3185,6 +3186,16 @@ parse_config_file(FILE *f, struct config *conf, const char *path, bool errors_ar
         errno = 0;
     }
 
+    struct color_theme *color_themes[] = {&conf->colors, &conf->colors2};
+    for (int i = 0; i < ALEN(color_themes); i++) {
+        struct color_theme *color_theme = color_themes[i];
+        if (color_theme->dim_blend_towards != DIM_BLEND_TOWARDS_INVALID)
+            continue;
+        color_theme->dim_blend_towards =
+            is_dark_theme(color_theme->fg, color_theme->bg)
+            ? DIM_BLEND_TOWARDS_BLACK : DIM_BLEND_TOWARDS_WHITE;
+    }
+
     if (errno != 0) {
         LOG_AND_NOTIFY_ERRNO("failed to read from configuration");
         if (errors_are_fatal)
@@ -3442,7 +3453,7 @@ config_load(struct config *conf, const char *conf_path,
             .flash_alpha = 0x7fff,
             .alpha = 0xffff,
             .alpha_mode = ALPHA_MODE_DEFAULT,
-            .dim_blend_towards = DIM_BLEND_TOWARDS_BLACK,
+            .dim_blend_towards = DIM_BLEND_TOWARDS_INVALID,
             .selection_fg = 0x80000000,  /* Use default bg */
             .selection_bg = 0x80000000,  /* Use default fg */
             .cursor = {
@@ -3538,7 +3549,6 @@ config_load(struct config *conf, const char *conf_path,
     memcpy(conf->colors.table, default_color_table, sizeof(default_color_table));
     memcpy(conf->colors.sixel, default_sixel_colors, sizeof(default_sixel_colors));
     memcpy(&conf->colors2, &conf->colors, sizeof(conf->colors));
-    conf->colors2.dim_blend_towards = DIM_BLEND_TOWARDS_WHITE;
 
     parse_modifiers(XKB_MOD_NAME_SHIFT, 5, &conf->mouse.selection_override_modifiers);
 
@@ -4096,6 +4106,30 @@ check_if_font_is_monospaced(const char *pattern,
     return is_monospaced;
 }
 
+pixman_color_t
+color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha)
+{
+    return (pixman_color_t){
+        .alpha = alpha,  /* Consider alpha linear already? */
+        .red = srgb_decode_8_to_16((color >> 16) & 0xff),
+        .green = srgb_decode_8_to_16((color >> 8) & 0xff),
+        .blue = srgb_decode_8_to_16((color >> 0) & 0xff),
+    };
+}
+
+// See https://en.wikipedia.org/wiki/Relative_luminance
+static float relative_luminance(uint32_t rgb)
+{
+    pixman_color_t srgb =
+        color_hex_to_pixman_srgb(rgb, /*alpha=*/UINT16_MAX);
+    return 0.2126 * srgb.red + 0.7152 * srgb.green + 0.0722 * srgb.blue;
+}
+
+bool is_dark_theme(uint32_t fg, uint32_t bg)
+{
+    return relative_luminance(bg) < relative_luminance(fg);
+}
+
 #if 0
 xkb_mod_mask_t
 conf_modifiers_to_mask(const struct seat *seat,

config.h

@@ -148,6 +148,7 @@ struct color_theme {
     enum {
         DIM_BLEND_TOWARDS_BLACK,
         DIM_BLEND_TOWARDS_WHITE,
+        DIM_BLEND_TOWARDS_INVALID,
     } dim_blend_towards;
 
     enum {
@@ -473,3 +474,7 @@ conf_modifiers_to_mask(
 #endif
 bool check_if_font_is_monospaced(
     const char *pattern, user_notifications_t *notifications);
+
+pixman_color_t
+color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha);
+bool is_dark_theme(uint32_t fg, uint32_t bg);

doc/foot-ctlseqs.7.scd

@@ -664,9 +664,7 @@ manipulation sequences. The generic format is:
 :  Query the current (color) theme mode
 :  contour
 :  The current color theme mode (light or dark) is reported as *CSI ?
-   997 ; 1|2 n*, where *1* means dark and *2* light. By convention, the
-   primary theme in foot is considered dark, and the alternative theme
-   light.
+   997 ; 1|2 n*, where *1* means dark and *2* light.
 
 
 # OSC

doc/foot.ini.5.scd

@@ -1138,17 +1138,12 @@ dark theme (since the default theme is dark).
 # SECTION: colors2
 
 This section defines an alternative color theme. It has the exact same
-keys as the *colors* section. The default values are the same, except
-for *dim-blend-towards*, which defaults to *white* instead.
+keys and default values as the *colors* section.
 
 Note that values are not inherited. That is, if you set a value in
 *colors*, but not in *colors2*, the value from *colors* is not
 inherited by *colors2*.
 
-In the context of private mode 2031 (Dark and Light Mode detection),
-the alternative theme (i.e. the *colors2* section) is considered to be
-the light theme (since the default, the primary theme, is dark).
-
 # SECTION: csd
 
 This section controls the look of the _CSDs_ (Client Side

foot.ini

@@ -132,7 +132,7 @@
 # bright7=ffffff   # bright white
 
 ## dimmed colors (see foot.ini(5) man page)
-# dim-blend-towards=black
+# dim-blend-towards=<black if dark mode else white>
 # dim0=<not set>
 # ...
 # dim7=<not-set>
@@ -171,8 +171,7 @@
 
 [colors2]
 # Alternative color theme, see man page foot.ini(5)
-# Same builtin defaults as [color], except for:
-# dim-blend-towards=white
+# Same builtin defaults as [color]
 
 [csd]
 # preferred=server

render.c

@@ -40,7 +40,6 @@
 #include "selection.h"
 #include "shm.h"
 #include "sixel.h"
-#include "srgb.h"
 #include "url-mode.h"
 #include "util.h"
 #include "xmalloc.h"
@@ -229,17 +228,6 @@ attrs_to_font(const struct terminal *term, const struct attributes *attrs)
     return term->fonts[idx];
 }
 
-static pixman_color_t
-color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha)
-{
-    return (pixman_color_t){
-        .alpha = alpha,  /* Consider alpha linear already? */
-        .red = srgb_decode_8_to_16((color >> 16) & 0xff),
-        .green = srgb_decode_8_to_16((color >> 8) & 0xff),
-        .blue = srgb_decode_8_to_16((color >> 0) & 0xff),
-    };
-}
-
 static inline pixman_color_t
 color_hex_to_pixman_with_alpha(uint32_t color, uint16_t alpha, bool srgb)
 {

terminal.c

@@ -1,4 +1,6 @@
 #include "terminal.h"
+#include "pixman.h"
+#include <stdint.h>
 
 #if defined(__GLIBC__)
 #include <malloc.h>
@@ -4785,10 +4787,6 @@ term_theme_toggle(struct terminal *term)
 /*
  * 1 - dark mode
  * 2 - light mode
- *
- * In foot, the themes aren't necessarily light/dark,
- * but by convention, the primary theme is dark, and
- * the alternative theme is light.
  */
 void term_send_color_theme_mode(struct terminal* term)
 {
@@ -4796,6 +4794,6 @@ void term_send_color_theme_mode(struct terminal* term)
         "\033[?997;2n",
         "\033[?997;1n",
     };
-    const bool is_dark = term->colors.active_theme == COLOR_THEME1;
+    const bool is_dark = is_dark_theme(term->colors.fg, term->colors.bg);
     term_to_slave(term, reply[is_dark], 9);
 }

tests/meson.build

@@ -1,6 +1,7 @@
 config_test = executable(
   'test-config',
   'test-config.c',
+  srgb_funcs,
   wl_proto_headers,
   link_with: [common, tokenize],
   dependencies: [pixman, xkb, fontconfig, wayland_client, fcft, tllist])